Elusive mergers of black holes with neutron stars confirmed for first time

For the first time, researchers have confirmed the detection of a collision between a black hole and a neutron star. In fact, the research team, which includes scientists from Penn State, detected not one but two such events occurring just 10 days apart in January 2020. The extreme events made splashes in space that sent gravitational waves rippling across at least 900 million light-years to reach Earth. In each case, the neutron star was likely swallowed whole by its black hole partner. 

Gravitational waves are disturbances in the curvature of space-time created by massive objects in motion. During the five years since the waves were first measured, a finding that led to the 2017 Nobel Prize in Physics, researchers have identified more than 50 gravitational-wave signals from the merging of pairs of black holes and of pairs of neutron stars. Both black holes and neutron stars are the corpses of massive stars, with black holes being even more massive than neutron stars.

Image

Now, in a new study, scientists have announced the detection of gravitational waves from two rare events, each involving the collision of a black hole and a neutron star. The gravitational waves were detected by the National Science Foundation's (NSF's) Laser Interferometer Gravitational-Wave Observatory (LIGO) in the United States and by the Virgo detector in Italy. The KAGRA detector in Japan, joined the LIGO-Virgo network in 2020, but was not online during these detections.

“With this new discovery, not only are we filling the missing piece for the zoo of compact object binaries, but also we can now study how many of such systems exist in the universe, where they have been formed and how often they merge,” said Leo Tsukada, a postdoctoral scholar at Penn State and a LIGO team member.

The first merger, detected on Jan. 5, 2020, involved a black hole about 9 times the mass of our sun, or 9 solar masses, and a 1.9-solar-mass neutron star. The second merger was detected on Jan. 15, and involved a 6-solar-mass black hole and a 1.5-solar-mass neutron star. Astronomers have spent decades searching for neutron stars orbiting black holes in the Milky Way, our home galaxy, but had found none prior to 2020 .  

The first of the two events, GW200105, was observed by the LIGO Livingston and Virgo detectors. It produced a strong signal in the LIGO detector but the signal was weak relative to noise from other sources in the Virgo detector. The other LIGO detector, located in Hanford, Washington, was temporarily offline. In general, detection of gravitational waves with only one detector can make it challenging to find an event with statistical significance.

“GW200105 is exceptional in the sense that it stands out against the background data collected across the first half of the observing run all the way up to the event time in all detectors. It is evidently different from features that we expect from noise, and that’s how we know that this must be a gravitational wave event.” said Divya Singh, a graduate student at Penn State and a LIGO team member. 

Given the nature of the gravitational waves, the team inferred that the signal was caused by a black hole colliding with a 1.9-solar-mass compact object, later identified as a neutron star. This merger took place 900 million light-years away. Because the signal was strong in only one detector, the location of the merger on the sky remains uncertain, lying somewhere in an area that is 34,000 times the size of a full moon.

The second event, GW200115, was detected by both LIGO detectors and the Virgo detector. “Unlike the earlier event, GW200115 doesn’t stand out as much against the background noise if we only look at one detector,” said Singh. “Therefore, GW200115 derives its higher confidence from coincident detection between all three detectors. “

GW200115 comes from the merger of a black hole with a 1.5-solar mass neutron star that took place roughly 1 billion light-years from Earth. Using information from all three instruments, scientists were better able to narrow down the part of the sky where this event occurred. Nevertheless, the localized area is almost 3,000 times the size of a full moon.

 

“Neutron-star, black-hole systems are fascinating physically because whether or not they emit light depends both on the properties of extremely dense matter and the properties of space and time near a spinning black hole.” said Chad Hanna, associate professor of physics and of astronomy and astrophysics at Penn State and a LIGO team member. 

Astronomers were alerted to both events soon after they were detected in gravitational waves and subsequently searched the skies for associated flashes of light. None were found. This is not surprising due to the very large distance to these mergers, which means that any light coming from them, no matter what the wavelength, would be very dim and hard to detect with even the most powerful telescopes. Additionally, the mergers likely did not give off a light show in any case because their black holes were big enough that they swallowed the neutron stars whole. 

The Penn State LIGO group played a crucial role in this new discovery. "Both of these mergers were detected as part of real-time gravitational wave processing conducted by members of the Penn State LIGO group,” said Hanna. “We get very excited when we detect a merger that may contain a neutron star because such systems are potential multi-messenger sources and might produce light or high energy sub-atomic particles - although these two mergers did not to our knowledge." 

Many of the follow-up analyses needed for the publication were performed using a computing cluster at the Penn State LIGO group. “It is very fascinating that our computers now serve as an essential resource for such a historical milestone in astrophysics,” said Tsukada. “Also personally it has been a great opportunity to be involved in its publication and paper-writing.”  

The results were published June 29 in The Astrophysical Journal Letters.

"The detector groups at LIGO, Virgo, and KAGRA are improving their detectors in preparation for the next observing run scheduled to begin in summer 2022," said Patrick Brady, a professor at University of Wisconsin-Milwaukee and spokesperson of the LIGO Scientific Collaboration. "With the improved sensitivity, we hope to detect merger waves up to once per day and to better measure the properties of black holes and super-dense matter that makes up neutron stars."

LIGO Research at Penn State is supported by the National Science Foundation and is conducted within the Institute for Gravitation and the Cosmos and the Institute for Computational and Data Sciences. Faculty, students and postdocs in the Penn State LIGO group are members of the physics and astronomy and astrophysics departments within the Eberly College of Science.

Additional information about the gravitational-wave observatories:

This material is based upon work supported by NSF's LIGO Laboratory which is a major facility funded by the NSF. LIGO is operated by Caltech and MIT, which conceived of LIGO and led the Advanced LIGO detector project. Financial support for the Advanced LIGO project was principally from the NSF with Germany (Max Planck Society), the U.K. (Science and Technology Facilities Council) and Australia (Australian Research Council-OzGrav) making significant commitments and contributions to the project. Approximately 1,400 scientists from around the world participate in the effort to analyze the data and develop detector designs through the LIGO Scientific Collaboration, which includes the GEO Collaboration. A list of additional partners is available at this link.

The Virgo Collaboration is currently composed of approximately 650 members from 119 institutions in 14 different countries including Belgium, France, Germany, Hungary, Italy, the Netherlands, Poland, and Spain. The European Gravitational Observatory (EGO) hosts the Virgo detector near Pisa in Italy, and is funded by the Centre National de la Recherche Scientifique (CNRS) in France, the Istituto Nazionale di Fisica Nucleare (INFN) in Italy, and Nikhef in the Netherlands. A list of the Virgo Collaboration groups can be found at this link. More information is available on the Virgo website.

The KAGRA detector is located in Kamioka, Gifu, Japan. The host institute is the Institute of Cosmic Ray Researches (ICRR) at the University of Tokyo, and the project is co-hosted by National Astronomical Observatory in Japan (NAOJ) and High Energy Accelerator Research Organization (KEK). KAGRA completed its construction in 2019, and later joined the international gravitational-wave network of LIGO and Virgo. The actual data-taking was started in February 2020 during the final stage of the run called "O3b." The KAGRA collaboration is composed of over 470 members from 11 countries/regions. The list of researchers is available at this link. More information is available at the KAGRA  website.